The present invention relates to an in phase quadrature signal generator used particularly in the field of transmissions, for applications such as image frequency rejection receivers or transmitters.
This signal generator includes phase shift means formed of passive elements for delivering in phase quadrature signals from a received input signal.
Such generators, also called I-Q phase shifters (In-phase and Quadrature), are known in the prior art. The general principle is described in FIG. 1 showing a generator whose phase-shifter 1 uses an RC-CR type passive filter stage. An input signal Vin is received at an input terminal 2 of the phase-shifter. This signal Vin is supplied to a first phase-shifter circuit 3 including a resistor R1 connected between the input terminal 2 and a first output terminal 5 of the generator and a capacitor C1 connected between said output terminal 5 and a reference potential, for example to the ground. The signal Vout—I delivered at output terminal 5 of this first phase-shifter circuit 3 is phase shifted with respect to the signal Vin supplied at the input. This signal Vin is supplied in parallel to a second phase shifter circuit 4 including a capacitor C2 connected between the input terminal 2 and a second output terminal 6 of phase shifter 1 and a resistor R2 connected between said second output and the ground. The signal Vout—Q delivered at output terminal 6 is also phase shifted with respect to the signal Vin supplied at the input. Signals Vout—I and Vout—Q are phase shifted by 90° with respect to each other, which is why one speaks of in phase quadrature signals.
Considering the output impedances Z1 and Z2 associated with output terminals 5 and 6, the following equation is obtained as a transfer function between output signals Vout—I and Vout—Q:       Vout_I    Vout_Q    =            1              j        ⁢                                  ⁢                  ω          ·          R2          ·          C2                      ·                  1        +                  j          ⁢                                          ⁢                      ω            ·            R2            ·            C2                          +                  R2          Z2                            1        +                  j          ⁢                                          ⁢                      ω            ·            R1            ·            C1                          +                  R1          Z1                    This type of generator is more advantageous when the in phase quadrature signals Vout—I and Vout—Q have the same amplitude. In order to achieve this, the transfer function explained hereinbefore has to have a module with a value of 1.
This type of circuit is integrated in wafers. Passive elements, like resistor or capacitance values, can be matched with an acceptable order of magnitude on the same wafer, of the order of one percent. This order of magnitude varies however, depending on the technology used.
Assuming a perfect match of the resistor values (R1=R2=R), capacitance values (C1=C2=C) and impedance values (Z1=Z2), the transfer function can be simplified as follows:       Vout_I    Vout_Q    =      1          j      ⁢                          ⁢              ω        ·        R        ·        C            The amplitudes of the output signals Vout—I and Vout—Q are thus equal for a single frequency f0, also called the cut-off frequency, whose corresponding angular frequency ω0 has a value of       1          R      ·      C        .
However, this type of in phase quadrature signal generator has several drawbacks. Indeed, as mentioned previously, matching of the passive elements of a circuit can only be achieved in an acceptable manner on a same wafer. For such circuits, dispersion between the passive elements from one wafer to another wafer is relatively significant, and can be of the order of ±30% for example for digital CMOS technology.
Thus the frequency f0, which directly depends on the values of these passive elements, varies greatly from one wafer to another, which raises serious problems for applications using a given working frequency.
Moreover, this dispersion represents a major drawback, in particular in application of the image frequency rejection receiver type. Within such applications, filtering means are provided for filtering the whole range of frequencies capable of containing the image frequency in order to retain only that of the signal generated at the working frequency.
Some solutions for avoiding the aforementioned drawbacks have already been proposed in the prior art. A first solution of the prior art consists in using a passive polyphase filter with several stages using different cut-off frequencies. The number of stages and the placing of the poles, defining the different cut-off frequencies, allow the quality of the image frequency rejection to be evaluated as a function of the mismatching of the components and their absolute value. This solution has the drawback of decreasing the amplitude of the signals at the output of each stage.
Another solution according to the prior art consists in making an active polyphase filter. Synthesis of this type of filter requires the use of several transconductors with a high dynamic range. The power consumption and surface area of these filters are significant.
In other applications where the signal amplitude is not significant, since it does not carry data, one can omit the amplitude error by using, for each of the quadrature signals, a large gain amplifier that limits the amplitude, but at the cost of high power consumption.
It will be noted that each of these solutions heavily penalises the circuit's power consumption and that the image frequency rejection remains limited in all cases because of the mismatching of the components used from one wafer to another.